1. Field of the Invention
The invention relates to a process for the synthesis of zeolites belonging to the faujasite structural group. It further relates to the products obtained and to their application in adsorption and catalysis.
2. Description of the Related Art
Zeolites are crystalline tectosilicates. The structures consist of assemblies of TO.sub.4 tetrahedra forming a three-dimensional skeleton by sharing oxygen atoms. In zeolites of the aluminosilicate type which are the most common ones, T denotes tetravalent silicon and trivalent aluminium. The abovementioned three-dimensional skeleton exhibits cavities and channels which have molecular dimensions and accommodate cations compensating the charge deficiency linked with the presence of trivalent aluminium in the TO.sub.4 tetrahedra, the said cations being generally exchangeable.
As a general rule, the composition of zeolites may be denoted by the empirical formula (M.sub.2 /.sub.n O; Y.sub.2 O.sub.3 ; x ZO.sub.2) in the dehydrated and calcined state. In this formula Z and Y denote the tetravalent and trivalent elements of the TO.sub.4 tetrahedra respectively, M denotes an electropositive element of valency n, such as an alkali or alkaline-earth metal and constitutes the compensating cation, and x is a number which can vary from 2 to theoretically infinity, in which case the zeolite is a silica.
Each type of zeolite has a distinct microporous structure. The variation in the dimensions and shapes of the micropores from one type to another results in changes in the adsorbent properties. Only molecules which have certain dimensions and shapes are capable of entering the pores of a particular zeolite.
Because of these remarkable characteristics, zeolites are very particularly suitable for the purification or separation of gaseous or liquid mixtures, such as, for example, the separation of hydrocarbons by selective adsorption.
The chemical composition, including in particular the nature of the elements present in the TO.sub.4 tetrahedra and the nature of the exchangeable compensating cations, is also an important factor involved in the selectivity of the adsorption, and above all in the catalytic properties of these products. They are employed as catalysts or catalyst supports in the cracking, reforming and modification of hydrocarbons, and in the conversion of many molecules.
Many zeolites exist in nature; they are alumino-silicates whose availabilities and properties do not always correspond to the requirements of industrial applications. Consequently, the search for products which have new properties has led to the synthesis of a large variety of zeolites, essentially of the aluminosilicate type. Among the many examples of this type there may be mentioned zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite L (FR-A-1,224,154), zeolite T (FR-A-1,223,775), zeolite ZSM5 (U.S. Pat. No. 3,702,886), zeolite ZSM 12 (U.S. Pat. No. 3,832,449) and zeolite ZSM 48 (EP-A-0,015,132).
Zeolites of the faujasite structural group are characterized by a three-dimensional framework structure which can be described by means of the assembly of modules called cube-octahedra. Each of these modules consist of 24 tetrahedra containing the elements Si and Al and bridged by oxygen according to the principle described above. In the cube-octahedron the tetrahedra are linked so as to form eight rings containing six tetrahedra and six rings containing four tetrahedra. Each cube-octahedron is joined with tetrahedral coordination, via four rings containing six tetrahedra, to four neighbouring cube-octahedra.
To show the relationships which unite the various members of the structural group it is convenient to consider the structural planes in which the cube-octahedra are arranged at the vertices of a plane network of hexagons. Each cube-octahedron is thus connected to three neighbours in the structural plane.
The fourth connecting direction is directed alternately on each side of the structural plane and enables the cube-octahedra to be connected between neighbouring and parallel structural planes.
According to the mutual relative arrangement of these structural planes, it is possible to obtain
sequences of three distinct structural planes ABCABC . . . corresponding to a structure of cubic symmetry, PA1 sequences of two distinct structural planes ABAB . . . corresponding to a structure of hexagonal symmetry, PA1 more complex sequences, which may be regular or irregular. PA1 crown ethers whose ring contains 10 to 24 atoms and comprises solely oxygen atoms as heteroatoms, at least 4 in number, among which the following compounds may be mentioned: PA1 compounds which have a structure comparable to that of the above crown ethers but in which the oxygen atoms in the ring are partially or completely replaced by substituents chosen from sulphur atoms and the groups &gt;NH, &gt;NR and ##STR1## in which R is a C.sub.1 -C.sub.4 hydrocarbyl, among which there may be mentioned the following compounds: PA1 carbon-containing macropolyrings of the type of polyoxadiazabicycloalkanes in which each ring contains 10 to 18 atoms and has at least two oxygen atoms in addition to the two nitrogen atoms, among which there may be mentioned the following compounds: PA1 chemical analysis PA1 radiocrystallography (cf. D. W. Breck: "Zeolite Molecular Sieves", publ. John Wiley and Sons, New York, 1974, page 94) PA1 silicon 29 NMR (cf. J. Klinowski: "Progress in NMR Spectroscopy", 1984, Vol. 16, pages 237 to 309). PA1 (vM.sub.1.sup.q+) (wM.sup.n+) ((SiO.sub.2).sub.192-x (AlO.sub.2).sub.x).sup.x- (z H.sub.2 O) (I) with, in this formula, M.sup.1.sub.q+ denoting a q-valent cation of a metal of group IA of the Periodic Classification of the Elements (q=1) or of an alkaline-earth metal chosen from Ca, Sr and Ba (q=2) or a monovalent cation containing nitrogen (q=1), especially ammonium or quaternary ammonium, M.sup.n+ denoting a metal cation of valency n other than a cation M.sub.1.sup.q+, x, z, w and v being numbers such that 30&lt;x.ltoreq.96, Z.gtoreq.0 according to the hydration state of the zeolite (z=0 for a completely anhydrous zeolite), 0&lt;v.ltoreq.x/q and 0.ltoreq.w.ltoreq.x/n with qv+wn.gtoreq.x.
All the solids belonging to the faujasite structural group are polytypes and have interconnected channels approximately 0.8 nm in diameter. Thus, faujasite is a natural zeolite whose structure corresponds to the stacking of three distinct structural planes ABC corresponding to a structure of cubic symmetry. Compounds with the same structure as faujasite can be obtained by synthesis from a sodium aluminosilicate gel, the said compounds being called zeolites X when the Si:Al ratio of the number of atoms of silicon to the number of atoms of aluminium is between 1 and 1.5, and zeolites Y when the said Si:Al ratio is between 1.5 and 3. Si:Al ratios higher than 3 cannot be obtained by synthesis.
Nevertheless, there exist postsynthesis treatments which make it possible to raise the value of the Si:Al ratio above 3, for example a high-temperature steam treatment after the Na.sup.+ cations have been exchanged for protons or lanthanumcations. Certain properties can thus be improved, such as the hydrothermal stability needed in certain applications like, for example, the cracking of hydrocarbon molecules in petroleum refining.
Compounds whose structure approaches the hexagonal structure ABABAB . . . can also be obtained by synthesis, but with stacking defects, which are seen as the broadening of some lines in the x-ray diffraction patterns employed to identify these compounds. Thus, zeolite ZSM-3 (U.S. Pat. No. 3,415,736) is prepared in a medium containing Na.sup.+ and Li.sup.+ cations; its Si:Al ratio is close to 1.5. By employing the Cs.sup.+ and Na.sup.+ cation pair, zeolite CSZ-3 (U.S. Pat. No. 4,333,859) is obtained, whose Si:Al ratio is close to 3. Zeolites of the type ZSM 20 (U.S. Pat. No. 3,972,983) crystallize in the presence of tetraethylammonium cations (TEA.sup.+) associated with Na.sup.+ cations; however, to obtain the maximum Si:Al ratio of 4.4, an aluminosilicate gel which has a Si:Al ratio close to 15 and a TEA.sup.+ :Si molar ratio close to 1 must be employed.
The general process of synthesis of zeolites of the faujasite structural group consists of a hydrothermal crystallization of aluminosilicate gels of particular compositions containing a structuring agent, which may be a metal cation and optionally an organic cation or compound such as TEA.sup.+.
More precisely, a process of this kind consists in producing first of all a reaction mixture which has a pH higher than 10 and contains water, a source of tetravalent silicon, a source of trivalent aluminium, a source of hydroxide ions in the form of a strong inorganic or organic base, optionally a source of M.sup.n+ metal cations, n being the valency of M, and optionally a structuring agent ST so as to obtain an aluminosilicate gel which has the desired composition to permit its crystallization into a compound of the faujasite structural group, and in then maintaining the gel obtained, directly or after prior maturing, at a temperature not exceeding 150.degree. C. and under a pressure which is at least equal to the autogenous pressure of the mixture consisting of the said gel for a sufficient period to effect the crystallization of this gel.
When the structuring agent is an organic compound such as TEA.sup.+, the product resulting from the crystallization, after washing with distilled or deionized water and drying below 100.degree. C., is a precursor of the required zeolite, which consists of the said zeolite trapping the structuring agent in its cavities. The change from the precursor to the corresponding zeolite is made by subjecting the said precursor to a calcination at a temperature which is suitable for destroying the organic structuring agent.
As indicated earlier, a process of this kind does not make it possible to synthesize zeolites which have the faujasite structure of cubic symmetry and a Si:Al ratio higher than 3. Moreover, in the case of the synthesis of hexagonal polytype zeolites of faujasite, obtaining a Si:Al ratio higher than 3 requires the use of an aluminosilicate gel exhibiting a high Si:Al ratio and simultaneously containing a molar quantity of structuring agent, for example TEA.sup.+ cations, which is close to the molar quantity of silica employed to form the initial reaction mixture, that is to say substantially higher than the molar quantity of the aluminium element.